The present invention relates to an active pixel sensor, and especially to an active pixel sensor operated in a low voltage environment. The power consumption of the active pixel sensor of the invention is lower than the power consumption of a conventional charge coupled device (CCD) image sensor, and also lower than the power consumption of the CMOS active pixel sensor which is now extremely developed by current commercial industry.
For many years, in the design of the image capturer and monitor, the CCD has been predominantly used due to the characteristics of lower noise, small size of pixel, etc. However, with the development of portable systems, the major concerns in the design of integrated circuits are low voltage (less cells) and low power (long working time), while in the application of CCD components, a higher voltage is needed to drive the system, and thus the difficulty in design is increased. Recently, because of the improvements in CMOS manufacturing processes, the size of device has become smaller, and thus the active pixel sensor (APS) has also further improved. Since the active pixel sensor has the advantages of lower working voltage and lower power, and is a high compact integrated circuit, it has a high commercial worth.
The conventional active pixel sensor is shown in FIG. 1. The primary characteristic thereof is that in each pixel, an amplifier circuit is used to output signals. Referring to FIG. 1, upon application of a control signal RST, the diode D1 is charged by a reverse bias voltage. During exposing, the diode D1 may be considered as an equivalent current source to charge point g0. The transistor M2 is an amplifying stage of an output amplifier. The voltage stored by point g0 is output to BL. The output of the pixel signal may then be controlled by a control signal WL. However, if the working voltage is reduced (for example, to 2V), due to the body effect, the bias voltage on point g0 will become insufficient, and thus it can not be operated under a lower voltage environment.